Field of the Invention
This invention relates to data display apparatus and more particularly to display apparatus in which characters are displayed on the screen of a cathode ray tube.
Characters may be displayed on a cathode ray tube screen by causing the beam of the cathode ray tube to repetitively scan the screen in the manner employed in television. A horizontal deflection circuit causes the beam to sweep horizontally across the screen at a substantially uniform rate, the beam being rapidly returned to the starting point at the end of each sweep. At the same time a vertical deflection circuit causes the beam to sweep vertically down the screen. The vertical sweep is at a much lower rate than the horizontal sweep so that a large number of horizontal sweeps occur for each vertical sweep. Hence the beam traces a large number of lines across the screen, each line being displaced vertically from the preceding line. During scanning of the screen a video modulation signal is applied to the cathode ray tube so that the beam is increased in intensity each time that it is swept past any point of the screen at which portions of those characters which it is desired to display are situated and thus a plurality of dots on the screen are illuminated by the electron beam and these dots together form the outlines of the required characters. During the remainder of the scanning of the screen the electron beam is cut off so that the remainder of the screen is not illuminated.
Normally the displayed characters are arranged in horizontal rows and a group, for example seven, of successive horizontal line sweeps are utilised to display a row of characters. In order to space one row of characters from a preceding row of characters, it is necessary to ensure that at least one line of the horizontal sweep between successive groups of horizontal sweeps is not modulated. If only a single horizontal line sweep is used for spacing between adjacent rows of characters it has been found that the spacing between the rows is inadequate. Adequate spacing can be obtained by utilising more than one line sweep for row spacing but then the number of rows of characters which could be displayed with a particular scanning arrangement would be reduced.